1. Field of the Invention
The present invention relates to data communications, and more specifically to a receiver which can efficiently recover the data encoded in a high speed serial communication channel. The invention has particular application in digital display units such as flat-panel monitors.
2. Related Art
Receivers are often used to recover data received on a serial communication channel. In a typical scenario, an encoder encodes information (data) in the form of a sequence of symbols and a modulator generates a signal encoding the sequence of symbols in a serial communication channel. The receiver receives the signal and recovers the encoded symbols. Once the symbols are recovered, the information represented by the symbols may then be easily generated.
To recover the symbols encoded in a received signal, receivers often oversample the received signal to generate multiple samples per each symbol. Oversampling generally refers to sampling a signal more number of times than the number of symbols encoded in the signal. Typical receivers include a phase picker to select from among the samples, with the selected samples representing the symbols encoded in the received signal. Assuming for illustration that an input signal is oversampled by a factor of L (L being a positive integer), phase pickers are generally designed to select one out of L samples.
A prior receiver may determine an optimal sampling phase for a group of successive symbols, and select samples according to the optimal sampling phase. For example, an average sampling phase may be determined and be used as the optimal sampling phase. Such an approach is generally simple to implement, and may therefore be suitable in many situations.
However, such approaches may not be suitable in some environments. For example, due to conditions such as noise and channel mis-equalization, the symbol boundaries may get shifted. If the selection of samples is based solely on an optimal sampling phase (for a group of successive symbols), the selected samples may not accurately represent the encoded symbols due to the shifts.
The probability of such inaccuracies generally increases as the ratio of frequency of encoding to the bandwidth of the transmission medium is higher. Thus, in may media having limited bandwidth, when the symbols are encoded at high frequency, short shifts in the boundaries may lead to a symbol being skipped or more than one sample of a symbol being selected. In other words, if the boundaries shift to make the corresponding symbol period (the duration in which a symbol is encoded) short, the samples from a corresponding symbol may be skipped altogether. On the other hand if the symbol period is long, more than one sample may be selected for a corresponding symbol. Both the cases may be unacceptable at least in some situations.
One prior approach may increase the oversampling factor and examine the samples to determine the optimal samples representing the encoded data. However, receivers based on such high oversampling factor may require additional electrical power and also may result in increased overall cost to design and manufacture. At least in markets targeted for consumer markets, the increased costs and power requirements may not be acceptable.
Therefore, what is needed is a method and apparatus which enables a receiver to accurately recover data encoded in a serial communication channel at least while minimizing the cost and power requirements.
The present invention enables a receiver to accurately recover the information encoded in a symbol stream received over a serial communication channel. The present invention is particularly useful in environments in which the samples are encoded at high baud rate. A receiver in accordance with the present invention may include an ADC (or multiple ADCs viewed logically as a single ADC) to oversample a received signal according to sampling clock signal to generate multiple samples.
A transition detector may generate transition indicators, with each transition indicator indicating the presence of a change in values of two successive samples. A static phase determination circuit may determine a static phase representing a long term phase shift of the signal relative to a sampling clock signal, wherein the long term phase shift of the signal is determined based on many prior samples corresponding to prior symbols.
A tokens analyzer may examine the transition indicators corresponding to a few symbols including a current symbol to determine any short term phase shift of boundaries between symbols around the present symbol on a per symbol basis. The token analyzer may determine which sample represents the current symbol according to the long term phase shift and the short term phase shift. A samples selector may select the sample determined by the token analyzer as representing the current symbol.
By considering the short term phase shifts, the present invention enables the samples to be selected accurately even in the presence of symbol period changes for individual symbols. Potentially, the samples selection may be made on a per symbol basis.
The receiver may further contain a token assembler for dividing the transition indicators into multiple tokens, with each token containing a number of transition indicators equal to a oversampling factor. Each token is associated with a symbol and the token corresponding to the current symbol is determined by the sampling clock signal.
The tokens analyzer and the static phase determination circuit are designed to examine the tokens corresponding to the few symbols to determine any phase shift in boundaries relative to the sampling clock signal, and use the determination as to shift in boundaries in computing the static phase. The static phase determination circuit may indicate whether the signal is early, late or neutral relative to the determination of the sampling clock signal.
The static phase determination circuit is designed to generate xe2x80x9chard identifiersxe2x80x9d if the examination of tokens corresponding to the few symbols indicates that the signal is early, late or neutral relative to the sampling clock signal, and to generate xe2x80x9csoft identifiersxe2x80x9d if the examination of tokens corresponding to the few symbols indicates that the signal is not early, not late or not neutral. As the hard identifiers generally provide more deterministic information on the relative phase shift, the hard identifiers are given more weight than the soft identifiers in determining the static phase. In one embodiment, the soft identifiers may be ignored.
A receiver in accordance with the present invention may be used in a digital display unit, which the symbols are encoded with an alphabet containing two elements (0 and 1). The transition detector may contains multiple XOR gates to generate an XOR of two successive samples. The signal may be oversampled by a factor of 3, and the present invention allows the samples to be selected accurately even if only a single sample is generated for a symbol due to, for example, jitter in the sampling clock or noise otherwise. In general, the present invention allows accurate recovery of a symbol even if the sampling clock is shifted 0 to L-1 samples.
In addition, the tokens analyzer may be implemented to determine the specific sample to select for a symbol by examining a single token (corresponding to the current symbol) and the static phase. Due to the minimal processing required, the present invention is particularly suited for environments in which the symbols are encoded at high baud rates.
Therefore, the present invention provides a receiver which can accurately recover the symbols received in a serial communication channel as the specific sample to be selected may be determined potentially on a per-symbol basis.
The present invention is particularly suitable for environments encoding symbols at high baud rates as the specific samples to be selected can be determined without requiring extensive processing.
The present invention allows for speedy recovery of the symbols as the static phase status may be computed in parallel to the selection of samples on a per-symbol basis.
The present invention allows for speedy recovery of the symbols as the computation of the status phase may enable the samples to be selected with minimal lookup and/or processing, for example, in the case of oversampling of 3, only the comparison points related to the present symbol may need to be examined.
The present invention is particularly useful in display units which receive signals encoded with pixel data elements at high frequencies as the encoded data can be recovered accurately in accordance with the present invention.
The present invention allows for accurate recovery of the symbols encoded in a serial communication channel as the boundary information around a current symbol is considered in determining the specific sample to be selected.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.